Drop Python 3.9 support

[jameslamb]

Description

Contributes to rapidsai/build-planning#88

Finishes the work of dropping Python 3.9 support.

This project stopped building / testing against Python 3.9 as of rapidsai/shared-workflows#235.
This PR updates configuration and docs to reflect that.

Notes for Reviewers

How I tested this

Checked that there were no remaining uses like this:

git grep -E '3\.9'
git grep '39'
git grep 'py39'

And similar for variations on Python 3.8 (to catch things that were missed the last time this was done).

[jameslamb]

/merge